Programmable polarity module for DC resistance spot welding

ABSTRACT

A programmable polarity module that permits rapid on-demand control of the polarities assigned to the welding electrodes retained on a welding gun is disclosed. The programmable polarity module is electrically connectable to the welding gun and a direct current power supply unit to provide direct current to the welding electrodes for exchange during spot welding. A first interchangeable polarity output lug and a second interchangeable polarity output lug of the programmable polarity module permit the polarities of the welding electrodes to be switched without having to electrically disconnect the module from the welding gun.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of U.S. patent application Ser. No.14/174,339, filed on Feb. 6, 2014, which claims the benefit of U.S.provisional patent application No. 61/774,227, filed on Mar. 7, 2013.The entire contents of each of the aforementioned applications areincorporated herein by reference.

TECHNICAL FIELD

The technical field of this disclosure relates generally to aprogrammable polarity module that can be connected to a direct current(“DC”) power supply for a resistance spot welding gun. The programmablepolarity module allows the polarity of the welding gun's electrodes tobe controlled, as needed, to best accommodate the resistance spotwelding process being practiced at the time.

BACKGROUND

Resistance spot welding is a process used by a number of industries tojoin together two or more metal workpieces. The automotive industry, forexample, often uses resistance spot welding to join togetherpre-fabricated sheet metal workpieces during the manufacture of avehicle door, hood, trunk lid, or lift gate, among others. A number ofspot welds are typically formed along a peripheral edge of the metalworkpieces or some other bonding region to ensure the part isstructurally sound. The most common metal workpieces used today in theautomotive industry are those made of steel and an aluminum alloy. Thedesire to incorporate aluminum alloys into a vehicle has made itenviable to spot weld an aluminum alloy workpiece to another aluminumalloy workpiece or, alternatively, to a steel workpiece.

The resistance spot welding process is performed by an automated roboticor pedestal welding gun that includes two arms. Each of these arms holdsa welding electrode typically comprised of a suitable copper alloy. Thewelding gun arms can be positioned on opposite sides of a workpiecestack-up and clamped to press the two electrodes against theirrespective metal workpieces at diametrically common spots. A momentaryelectrical current is then passed through the metal workpieces from oneelectrode to the other. Resistance to the flow of electrical currentthrough the metal workpieces and their faying interface (i.e., thecontacting interface of the metal workpieces) generates heat at thefaying interface. This heat forms a molten weld pool which, uponstoppage of the current flow, solidifies into a weld nugget. After thespot weld is formed, the welding arms release their clamping force, andthe spot welding process is repeated at another weld site.

The electric current that is passed between the opposed electrodes andthrough the metal workpieces is received from a DC power supply carriedby the welding gun. The DC power supply may, for example, be amedium-frequency integrated transformer and rectifier package configuredto deliver high DC amperage in accordance with a specified weldschedule. This type of DC power supply, and other similar types as well,furnishes the opposed electrodes with fixed opposite polarities whenelectrically connected to the welding gun; that is, after the DC powersupply has been installed, one electrode is always the positiveelectrode and the other is always the negative electrode.

The polarity assigned to the welding electrodes is not inconsequential.It has been found, for instance, that a polarity bias exists when spotwelding (1) an aluminum alloy workpiece to another aluminum alloyworkpiece, and (2) an aluminum alloy workpiece to a steel workpiece. Aless pronounced polarity bias also exists when spot welding a steelworkpiece to another steel workpiece and in certain practices ofprojection welding. The ability to control which electrode has thepositive/negative polarity while the welding gun and the DC power supplyremain electrically connected—including the ability to switch electrodepolarities at any time—would permit more operationally effective spotwelding practices to be developed in at least these instances, andpossibly others. Such electrode polarity control cannot be achieved withconventional DC power supplies. In fact, when a conventional DC powersupply is employed, the only way to change the polarity of theelectrodes is to physically disconnect the power supply from the weldinggun, and then re-connect the power supply in reverse polarityorientation, which is a time-consuming and laborious process.

SUMMARY

A programmable polarity module that permits rapid on-demand control ofthe polarities assigned to the welding electrodes retained on a weldinggun is disclosed. The programmable polarity module is electricallyconnectable to the fixed polarity output lugs of a DC power supply inany known fashion to provide a multi-component DC power supply unit. Itis also electrically connectable to the welding gun, and thus thewelding electrodes, by way of a first interchangeable polarity outputlug and a second interchangeable polarity output lug. The first andsecond interchangeable polarity output lugs can assign either a positivepolarity or a negative polarity to their associated welding electrodes.

Each of the first and second interchangeable polarity output lugs isassociated with a pair of high-amperage silicon controlled rectifiers(SCR's). Within each pair of SCRs, one SCR is associated with a positivepolarity and the other SCR is associated with a negative polarity. Thepairs of SCR's can thus be controlled to assign each interchangeableoutput polarity lug—and the welding electrode associated with eachlug—with a positive polarity or a negative polarity. This type ofcontrol permits the polarity designations of the two welding electrodesto be dictated in any conceivable way so that the particulars of avariety of spot welding processes can be accommodated. The polarity ofeach welding electrode can even be rapidly switched without having todisconnect the DC power supply from the welding gun.

The term “high-amperage silicon controlled rectifier” and itsabbreviation, “SCR,” as used herein, are meant to broadly encompass asingle thyristor or an arrangement of one or more thyristors that act intandem. Thyristors are electrical switching devices that includealternating p-type and n-type semiconductor layers that can becontrolled to permit or block current flow based on the magnitude (orlack thereof) of a voltage applied to a control terminal (also known asa gate). The number of thyristors employed in each SCR depends on themagnitude of the current that needs to be managed through the first andsecond interchangeable polarity output lugs. For example, each SCR inthe pairs of SCR's associated with the first and second interchangeablepolarity output lugs may be a single thyristor or, if the currentcapacity of a single thyristor is not sufficient for whatever reason, anarrangement of several thyristors connected in parallel that, together,can accommodate the magnitude of the current that needs to becontrolled.

The programmable polarity module can be used to cure the effects of anelectrode polarity bias that exists within a resistance spot weldingprocess. For example, when spot welding an aluminum alloy workpiece toanother aluminum alloy workpiece with a pair of copper alloy electrodes,the current exchanged between the welding electrodes may create a heatdifferential at the electrode/workpiece interfaces due to the flow ofelectrons across the aluminum alloy-copper alloy junctions.Specifically, more heat may be generated at the positive weldingelectrode than at the negative welding electrode, which causes thepositive welding electrode to wear at a faster rate. The programmablepolarity module could be used here to switch the polarities of the twoelectrodes every so often, preferably after every spot weld, to keep oneelectrode from wearing faster than the other.

As another example, an electrode polarity bias may exist when spotwelding dissimilar metal workpieces with a pair of copper alloyelectrodes. The dissimilar metal workpieces may be a pair of aluminumalloy sheet metal layers of considerably different thicknesses, or analuminum alloy sheet metal layer and an aluminum alloy casting, or analuminum alloy workpiece and a steel workpiece, to name but a few. Thespot welding of such dissimilar metal workpieces, like before, maycreate a heat imbalance at the electrode/workpiece interfaces in whichmore heat is generated at the positive welding electrode and less heatis generated at the negative welding electrode. Better quality spotwelds can generally be achieved by using this heat differential tooffset differences in the electrical conductivities and/or the meltingpoints of the dissimilar metal workpieces. The programmable polaritymodule could be used here to ensure that the welding electrodes areassigned the polarity that results in the best weld quality.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a generalized illustration of a welding gun for use inresistance spot welding applications;

FIG. 1A is a magnified view of the encircled portion of FIG. 1identified as 1A;

FIG. 2 is a generalized illustration of a welding electrode that can beused to perform resistance spot welding;

FIG. 3 is a generalized illustration of a DC power supply unit, whichincludes a DC power supply and a programmable polarity module, that canbe carried by the welding gun shown in FIG. 1;

FIG. 4 is a schematic illustration of the programmable polarity moduleillustrated in FIG. 3;

FIG. 5 is a picture of a welding electrode that has been provided with anegative polarity during repeated spot welding of an aluminum alloysheet metal layer to another aluminum alloy sheet metal layer;

FIG. 6 is a picture of a welding electrode that has been provided with apositive polarity during repeated spot welding of an aluminum alloysheet metal layer to another aluminum alloy sheet metal layer;

FIG. 7 is a cross-sectional photomicrograph of a resistance spot weldformed between an aluminum alloy sheet metal layer and a steel sheetmetal layer in which the welding electrode that engaged the aluminumalloy sheet metal layer had the negative polarity;

FIG. 8 is a cross-sectional photomicrograph of a resistance spot weldformed between an aluminum alloy sheet metal layer and a steel sheetmetal layer in which the welding electrode that engaged the aluminumalloy sheet metal layer had the positive polarity;

FIG. 9 is a cross-sectional photomicrograph of a resistance spot weldformed between an aluminum alloy sheet metal layer and an aluminum alloycasting in which the welding electrode that engaged the aluminum alloycasting had the negative polarity; and

FIG. 10 is a cross-sectional photomicrograph of a resistance spot weldformed between an aluminum alloy sheet metal layer and an aluminum alloycasting in which the welding electrode that engaged the aluminum alloycasting had the positive polarity.

DETAILED DESCRIPTION

FIGS. 1-1A generally depict a welding gun 10 that can be used toresistance spot weld a metal workpiece stack-up 12 at one or morepredetermined spot weld sites 14. The workpiece stack-up 12 includes,for example, a first metal workpiece 16 and a second metal workpiece 18.These metal workpieces 16, 18 overlap one another to provide a fayinginterface 20 at the weld site 14 where the spot welding process forms aweld nugget 22 that metallurgically joins the metal workpieces 16, 18together. The term faying interface 20, as used herein, encompassesinstances of direct overlapping contact between the workpieces 16, 18 aswell as instances where the workpieces 16, 18 may not be touching, butare nonetheless overlapping in close proximity to one another, such aswhen a thin layer of adhesive, sealer, or some other intermediatematerial is present. Each of the first and second metal workpieces 14,16 may have a thickness 160, 180 that ranges from about 0.3 mm to about6.0 mm, and preferably ranges from about 0.6 mm to about 3.0 mm, at theweld site 14.

At least one of the first or second metal workpieces 16, 18 is composedof an aluminum alloy. The aluminum alloy may be an aluminum-magnesiumalloy, an aluminum-silicon alloy, an aluminum-magnesium-silicon alloy,or an aluminum-zinc alloy. Some specific aluminum alloys of this kindare 5754 aluminum-magnesium alloy, 6022 aluminum-magnesium-siliconalloy, and 7003 aluminum-zinc alloy. The other of the first or secondmetal workpieces 16, 18 may be composed of an aluminum alloy, like theones just mentioned, or it may be composed of steel. The steel may be alow carbon steel, a galvanized low carbon steel, or a galvanizedadvanced high strength steel (AHSS). Some specific steels of this kindinclude interstitial-free (IF) steel, dual-phase (DP) steel,transformation-induced plasticity (TRIP) steel, and press-hardened steel(PHS). The term “metal workpiece” and its aluminum alloy and steelvariations are used broadly in the present disclosure to include a sheetmetal layer, a casting, an extrusion, and other aluminum alloy or steelpieces that are resistance spot weldable.

It should be noted that the weld nugget 22 shown in FIG. 1A is a genericillustration that is meant to be representative of the wide variety ofweld nugget compositions—and weld nugget locations—that can be formed atthe faying interface 20 of first and second metal workpieces 16, 18. Forexample, if the first and second metal workpieces 16, 18 are aluminumalloy sheet metal layers, the weld nugget formed at the faying interfaceof the two layers will penetrate into each layer to some extent. Atypical penetration depth of the weld nugget into each aluminum alloysheet metal layer is about 10% to about 80% of the thickness of thelayer. As another example, if the first metal workpiece 16 is a steelsheet metal layer and the second metal workpiece 18 is an aluminum alloysheet metal layer, the weld nugget formed at the faying interface of thetwo layers will penetrate mainly into the aluminum alloy sheet metallayer, primarily because aluminum alloys melt at a significantly lowertemperature than steel. The weld nugget 22 depicted in FIG. 1A is thusintended to represent a more inclusive variety of weld nugget and weldnugget locations than what is generically shown, including the specificexamples described here as well as any others that may be formed betweenthe different combinations of workpiece materials that can be employed.

The welding gun 10 includes a first gun arm 24 and a second gun arm 26.The first gun arm 24 includes a shank 28 that retains a first weldingelectrode 30. Likewise, the second gun arm 26 includes a shank 32 thatretains a second welding electrode 34. The first and second gun arms 24,26 may be stationary (pedestal welder) or robotically moveable, as iscustomary in the art, and are operated during spot welding to press thefirst and second welding electrodes 30, 34 against oppositely-facingsurfaces 36, 38 of the first and second metal workpieces 16, 18 indiametric alignment with one another at the weld site 14. The clampingforce assessed by the gun arms 24, 26 establishes good mechanical andelectrical contact between the welding electrodes 30, 34 and theirrespective engaged metal workpiece surfaces 36, 38.

The first and second welding electrodes 30, 34 are preferablywater-cooled copper alloy welding electrodes that include a body 40 anda weld face 42 at the end of the body 40, as illustrated in FIG. 2. Theweld face 42 is the part of the electrode that contacts the surface 36,38 of the metal workpiece 16, 18 being engaged by the electrode 30, 34.And it may incorporate any of a wide variety of designs that aresuitable for spot welding an aluminum alloy workpiece or a steelworkpiece. If the welding electrode 30, 34 is intended to engage analuminum alloy workpiece, for example, the weld face 42 is preferablydomed, as shown in FIG. 2, and may further be smooth, textured, orinclude surface features such as protruding ringed ridges. Some examplesof these types of copper alloy welding electrodes are described in U.S.Pat. Nos. 6,861,609, 8,222,560, 8,274,010, and 8,436,269, and U.S.Patent Application Publication No. 2009/0255908. If the weldingelectrode 30, 34 is intended to engage a steel workpiece, the weld face42 is preferably flat or domed as has long been known in the art. Anelectrode weld face design that may be used to weld both aluminum alloyand steel workpieces is described in U.S. Pat. No. 8,525,066.

A DC power supply unit 44, as shown best in FIG. 3, is carried by thewelding gun 10. The DC power supply unit 44 supplies a direct currentfor passage between the welding electrodes 30, 34 when they are pressedagainst the oppositely-facing surfaces 36, 38 of their respective metalworkpieces 16, 18. This supplied DC is sufficient to initiate a weldpool at the faying interface 20 according to a defined weld schedule.Additionally, the DC power supply unit 44 allows the polaritydesignations of the first and second welding electrodes 30, 34 to becontrolled. And it can perform these functions while remaining installedon the welding gun 10. The DC power supply unit 44 includes, as shown, aDC power supply 46 and a programmable polarity module 48.

The DC power supply 46 is configured to receive, for example, an inputsingle-phase medium frequency (˜1000 Hz) alternating current (AC) from aweld control (not shown), and to convert that input AC into ahigher-amperage welding DC, typically between about 5 kA and about 65kA, that is supplied to the welding gun 10. The DC power supply 46 maybe any known type that is suitable for conducting resistance spotwelding. For example, as shown in FIG. 3, the DC power supply 46 may bea water-cooled, medium-frequency DC power supply that includes, as anintegrated package, a transformer 50 and a rectifier 52. This type of DCpower supply is commercially available from a number of suppliersincluding ARO Welding Technologies (US headquarters in ChesterfieldTownship, Mich.) and Bosch Rexroth (US headquarters in Charlotte, N.C.).Other types of DC power supplies may of course be used, including thoseconfigured to receive a single-phase 60 Hz AC.

The transformer 50 receives the input AC at an input port 54. The inputAC is fed through a primary winding and is “stepped down” to create alower voltage, higher amperage secondary AC in a secondary winding. Thissecondary AC is then fed to the rectifier 52 where a collection ofsemiconductor diodes converts it into the welding DC. The rectifier 52includes a fixed positive polarity output lug 56 and a fixed negativepolarity output lug 58 that are composed of copper, a copper alloy, orsome other highly electrically conductive material. These output lugs56, 58 deliver the welding DC from the rectifier 52. Skilled artisanswill know and understand the function and operation of the transformer50 and the rectifier 52 and, as such, a more comprehensive descriptionof these two components and their integration into a single package neednot be provided here.

The programmable polarity module 48 is electrically connectable to therectifier 52 of the DC power supply 46. Here, as shown in FIGS. 1 and 3,the programmable polarity module 48 includes a pair of fixed polarityinput lugs 60. Each of these input lugs 60 is composed of copper, acopper alloy, or some other highly electrically conductive material. Oneof the input lugs 60 is electrically connectable to the positivepolarity output lug 56 of the rectifier 52 and the other is electricallyconnectable to the negative polarity output lug 58 of the rectifier 52.When electrically connected, as is the case in FIG. 1, the input lugs 60assume the polarity of whichever output lug 56, 58 they are associatedwith—i.e., the input lug 60 connected to the fixed positive polarityoutput lug 56 is afforded a positive polarity (designated positive lug602) and the input lug 60 connected to the fixed negative polarityoutput lug 58 is afforded a negative polarity (designated negative lug604). Bolting or any other suitable type of connection features may beused to physically fasten the programmable polarity module 48 and the DCpower supply 46 together.

The programmable polarity module 48 is also electrically connectable tothe welding gun 10 so that the welding DC can be delivered to thewelding electrodes 30, 34. The programmable polarity module 48 mayinclude a first interchangeable polarity output lug 62 and a secondinterchangeable polarity output lug 64 to facilitate such a connection.The first interchangeable polarity output lug 62 is electricallyconnectable to a first bus bar 66 and the second interchangeablepolarity output lug 64 is electrically connectable to a second bus bar68. The first and second bus bars 66, 68 are composed of copper, acopper alloy, or some other highly electrically conductive material, andare configured to electrically communicate with the first and second gunarms 24, 26 and ultimately the first and second welding electrodes 30,34, respectively. Like before, bolting or any other suitable type ofconnection features may be used to physically fasten the programmablepolarity module 48 and the welding gun 10 together.

The polarities of the first and second interchangeable polarity outputlugs 62, 64 are not fixed; rather, they can be switched between positiveor negative at any time in accordance with any conceivable welding plan.The ability to switch the polarity of the first and secondinterchangeable polarity output lugs 62, 64 ultimately permits thepolarities of the first and second welding electrodes 30, 34 to beswitched in a corresponding way. This is because the designated polarityof the first and second interchangeable polarity output lugs 62, 64establishes a matching polarity of the first and second weldingelectrodes 30, 34. For instance, if the first interchangeable polarityoutput lug 62 is designated positive and, consequently, the secondinterchangeable polarity output lug 64 is designated negative, then thefirst and second welding electrodes 30, 34 will be designated positiveand negative, respectively, until the polarities of the lugs 62, 64 areswitched. And when the polarities of the output lugs 62, 64 areswitched, the polarities of the welding electrodes 30, 34 will beswitched as well in the same way.

A circuit design that may be incorporated into the programmable polaritymodule 48 to switch the polarities of the first and secondinterchangeable polarity output lugs 62, 64 is shown schematically inFIG. 4. As shown, a first pair 70 of SCR's (silicon controlledrectifiers) is associated with the first interchangeable polarity outputlug 62 and a second pair 72 of SCR's is associated with the secondinterchangeable polarity output lug 64. The first pair 70 of SCR'sincludes a forward positive polarity SCR 74 and a reverse negativepolarity SCR 76. Similarly, the second pair 72 of SCR's includes aforward negative polarity SCR 78 and a reverse positive polarity SCR 80.The forward positive polarity SCR 74 and the reverse positive polaritySCR 80 are associated with one of the fixed polarity input lugs 60, andthe reverse negative polarity SCR 76 and the forward negative polaritySCR 78 are associated with the other input lug 60. It should bereiterated that, even though FIG. 4 shows the several SCR's 74, 76, 78,80 as a single thyristor, each of the forward positive polarity SCR 74,the reverse negative polarity SCR 76, the forward negative polarity SCR78, and the reverse positive polarity SCR 80 may also be an arrangementof one or more thyristors connected in parallel such that they act intandem to achieve the same cumulative function as a single thyristorwould, but with the added possibility of greater current capacity.

Each of the SCR's 74, 76, 78, 80 includes a gate 740, 760, 780, 800.These gates 740, 760, 780, 800 can be controlled to turn theirrespective SCR's 74, 76, 78, 80 “on” (gated)—which means current canflow through the SCR—or “off” (ungated)—which means current cannot flowthrough the SCR. Whether the SCR's 74, 76, 78, 80 are turned “on” or“off” depends on whether a voltage is applied to their gates 740, 760,780, 800 that meets or exceeds a gate voltage, which is typicallyanywhere between about 1V-10V. To turn any of the SCR's 74, 76, 78, 80“on,” and to thus permit current flow, a voltage is applied to therelevant gate 740, 760, 780, 800 that is equal to or greater than therequired gate voltage. To turn any of the SCR's 74, 76, 78, 80 “off,”and to thus block current flow, no voltage (i.e., 0V) or a voltage thatis less than the gate voltage is applied to the relevant gate 740, 760,780, 800. A controller 82 may be incorporated into the circuit design tocontrol which SCR's are turned “on” or “off” at any given time. Thecontroller 82 may be a microcontroller of any known kind, and it mayinterface with the gates 740, 760, 780, 800 through conventionalcircuitry known to skilled artisans.

Two modes for turning the SCR's 74, 76, 78, 80 “on” and “off” areapplicable here: a forward polarity mode and a reverse polarity mode. Inthe forward polarity mode, the forward positive polarity SCR 74 and theforward negative polarity SCR 78 are turned “on” while the reversenegative polarity SCR 76 and the reverse positive polarity SCR 80 areturned “off” This mode coordinates the positive input lug 602 with thefirst interchangeable polarity output lug 62 and the negative input lug604 with the second interchangeable polarity output lug 64. Suchcoordination assigns a positive polarity to the first interchangeablepolarity output lug 62 and a negative polarity to the secondinterchangeable polarity lug 64 within the context of the electricalcircuit shown in FIG. 4.

The reverse polarity mode achieves the opposite effect at theinterchangeable polarity output lugs 62, 64. Specifically, in thereverse polarity mode, the reverse negative polarity SCR 76 and thereverse positive polarity SCR 80 are turned “on” while the forwardpositive polarity SCR 74 and the forward negative polarity SCR 78 areturned “off.” This mode coordinates the positive input lug 602 with thesecond interchangeable polarity output lug 64 and the negative input lug604 with the first interchangeable polarity output lug 62. Suchcoordination assigns a positive polarity to the second interchangeablepolarity output lug 64 and a negative polarity to the firstinterchangeable polarity output lug 62 within the context of theelectrical circuit shown in FIG. 4. Table 1 below summarizes the forwardpolarity mode and the reverse polarity mode as just described.

TABLE 1 Polarity of Forward Reverse Forward Reverse Polarity of FirstSecond Positive Negative Negative Positive InterchangeableInterchangeable Polarity Polarity Polarity Polarity Polarity OutputOutput Polarity Mode SCR SCR SCR SCR Lug Lug Forward ON OFF ON OFFPOSITIVE NEGATIVE Polarity Mode Reverse OFF ON OFF ON NEGATIVE POSITIVEPolarity Mode

A resistance spot welding process that implements the programmablepolarity module 48 will now be described with reference to FIGS. 1, 3,and 4. To begin, the metal workpiece stack-up 12 is located between thefirst and second welding electrodes 30, 34 so that the weld site 14 isgenerally aligned with the electrodes' 30, 34 opposed weld faces 42. Themetal workpiece stack-up 12 may be brought to such a location, as isoften the case when the gun arms 24, 26 are part of a stationarypedestal welder, or the gun arms 24, 26 may be robotically moved tolocate the electrodes 30, 34 relative to the weld site 14 of thestack-up 12. Once the stack-up 12 is properly located, the first andsecond welding arms 24, 26 converge to press the weld faces 42 of thefirst and second welding electrodes 30, 34 against the oppositely-facingsurfaces 36, 38 of the first and second metal workpieces at the weldsite 14.

The welding DC supplied by the DC power supply unit 44 is then passedbetween the first and second welding electrodes 30, 34 and through thefirst and second metal workpieces 16, 18 and across the faying interface20. Resistance to the concentrated flow of the welding DC through themetal workpieces 16, 18 and across the faying interface 20 generatesheat at the faying interface 20 at the weld site 14. This heat initiatesa molten weld pool at the faying interface 20 that penetrates into oneor both of the workpiece 16, 18 depending on the composition and natureof the workpieces 16, 18. Upon stoppage of the welding DC current, themolten weld pool solidifies into the weld nugget 22. The first andsecond welding electrodes 30, 34 are then retracted from their engagedsurfaces 36, 38 of the metal workpieces 16, 18. Next, the workpiecestack-up 12 is re-located between the first and second weldingelectrodes 30, 34 at a different weld site 14, or it is moved away sothat another stack-up 12 can be located for spot welding. More spotwelds are then formed in the same way.

The programmable polarity module 48 can designate the polarities of thefirst and second welding electrodes 30, 34 as needed to best suit theparticular spot welding process being performed. The programmablepolarity module 48 can assign a positive polarity to the first weldingelectrode 30 and a negative polarity to the second welding electrode 34,or vice versa, and can further switch the polarities of the first andsecond welding electrodes 30, 34 at any time. Such flexibility is madepossible by controlling which of the SCR's 74, 76, 78, 80 are turned“on” and which are turned “off.” Recall that in the forward polaritymode, for instance, the first interchangeable polarity output lug 62,and thus the first welding electrode 30, is assigned the positivepolarity while the second interchangeable polarity output lug 64, andthus the second welding electrode 34, is assigned the negative polarity.The opposite is true in the reverse polarity mode, in which the firstinterchangeable polarity output lug 62, and thus the first weldingelectrode 30, is assigned the negative polarity while the secondinterchangeable polarity output lug 64, and thus the second weldingelectrode 34, is assigned the positive polarity.

The programmable polarity module 48 may be useful when spot welding analuminum alloy workpiece to another aluminum alloy workpiece with a pairof copper alloy welding electrodes. The aluminum alloy workpieces couldbe, for example, a pair of aluminum alloy sheet metal layers, one ofwhich is about 3.0 mm thick or less at the weld site 14. They could alsobe, as another example, a pair of aluminum alloy castings, one of whichis about 3.0 mm thick or less at the weld site 14. It has been foundthat repeatedly forming spot welds between such aluminum alloyworkpieces with a conventional spot welding set-up—in which one weldingelectrode has a fixed positive polarity and the other welding electrodehas a fixed negative polarity—causes the positive welding electrode towear at a faster rate than the negative welding electrode. The positivewelding electrode may, in some instances, wear approximately twice asfast as the negative welding electrode over the course of forming 30-100spot welds.

The wear experienced at the two welding electrodes 30, 34 is theaccumulation of a hard metal reaction product on the weld face 42 thatis derived from a metallurgical reaction between the aluminum alloy ofthe metal workpiece and the copper alloy of the welding electrode. Theaccumulation of this hard metal reaction product may eventually spalland form pits in the weld face 42. To visually demonstrate this wearmechanism, FIGS. 5 and 6 show photomicrographs of a fixed negativepolarity copper alloy welding electrode and a fixed positive polaritycopper alloy welding electrode, respectively, that have been usedtogether to form 100 spot welds in a pair of overlapping 2 mm thickaluminum alloy sheet metal layers. The fixed positive polarity weldingelectrode (FIG. 6) has plainly experienced more aluminum alloy-copperalloy reaction product accumulation on its weld face. Because of this,the positive welding electrode needs to be periodically redressed toremove the hard metal reaction product, or replaced with a new weldingelectrode, more often than the negative welding electrode.

The programmable polarity module 48 can mitigate the above-describedpolarity bias by periodically switching the polarities of the first andsecond welding electrodes 30, 34. The polarities may be switched eachtime a certain number of spot welds have been performed. Preferably, thepolarities of the first and second welding electrodes 30, 34 areswitched after every 1-5 spot welds, and most preferably after everyspot weld. For example, the programmable polarity module 48 may beoperated in its forward polarity mode, in which the first weldingelectrode 30 is assigned the positive polarity and the second weldingelectrode 34 is assigned the negative polarity, and the welding DC maybe supplied to form a first spot weld. Then, after the welding DC hasstopped, the programmable polarity module 48 switches to its reversepolarity mode, in which the first welding electrode 30 is assigned thenegative polarity and the second welding electrode 34 is assigned thepositive polarity, and the welding DC may be supplied to form a secondspot weld. The programmable polarity module 48 may then switch back toits forward polarity mode, and so on. This back-and-forth switching ofthe electrode polarities will even out the rates at which the twowelding electrodes 30, 34 wear and, as a result, increase the amount ofspot welds that can be formed with the two electrodes 30, 34 relative tothe conventional fixed electrode polarity spot welding technique.

The programmable polarity module 48 may also be useful when the spotwelding of an aluminum alloy workpiece and a metal workpiece wouldcreate a heat imbalance in the workpieces that degrades weldability. Forexample, an aluminum alloy sheet metal layer and a steel sheet metallayer have different physical characteristics (e.g., melting points,thermal conductivities, hardness, etc.), and when trying to spot weldthe two sheet metal layers together with a pair of copper alloyelectrodes, a heat imbalance develops as current passes through them. Inthis case, a greater amount of localized heat is generated in the moreelectrically resistive steel than the less electrically resistivealuminum alloy. A heat imbalance may also develop when trying to spotweld different types of aluminum alloy workpieces—such as an aluminumalloy sheet metal layer and an aluminum alloy casting—with a pair ofcopper alloy electrodes. This is because an aluminum alloy castingtypically has a higher electrical resistivity than an aluminum alloysheet metal layer.

The spot welding of such workpieces, like before, creates a heatimbalance at the electrode/workpiece interfaces in which more heat isgenerated at the positive welding electrode and less heat is generatedat the negative welding electrode. It has been found that the weldquality between the metal workpieces can be affected by controlling theelectrode polarities and, by extension, the heat imbalance developed atthe welding electrodes 30, 34. When spot welding an aluminum alloy sheetmetal layer and a steel sheet metal layer, for instance, the ability toswitch the polarities of the welding electrodes 30, 34 allows for theelectrode heat imbalance to be used to compensate for the lowerelectrical resistivity and the lower melting point of the aluminum alloysheet metal layer. Either the positive welding electrode or the negativewelding electrode may engage the aluminum alloy sheet metal layer togenerate more or less heat, respectively, so as to obtain better weldnugget penetration, preferably approaching 50%, into the aluminum alloysheet metal layer. When spot welding an aluminum alloy sheet metal layerto an aluminum alloy casting, the differences in electricalresistivities can usually be counteracted by engaging the moreelectrically resistive aluminum alloy casting with the negative weldingelectrode, which experiences less heat generation at theelectrode/workpiece interface compared to the positive weldingelectrode.

FIGS. 7-10 visually demonstrate the effects that electrode polarity canhave on weld quality. FIGS. 7 and 8 are cross-sectional photomicrographsof a 1 mm thick aluminum alloy sheet metal layer and a 0.55 mm thicksteel sheet metal layer that have been subjected to spot welding. FIG. 7shows the effect of engaging the aluminum alloy sheet metal layer(bottom layer) with a copper alloy welding electrode that has beenassigned the negative polarity and engaging the steel sheet metal layer(top layer) with a copper alloy welding electrode that has been assignedthe positive polarity. Conversely, FIG. 8 shows the effect of engagingthe aluminum alloy sheet metal layer (top layer) with a copper alloywelding electrode that has been assigned the positive polarity andengaging the steel sheet metal layer (bottom layer) with a copper alloywelding electrode that has been assigned the negative polarity. As canbe seen, in this particular example, engaging the aluminum alloy sheetmetal layer with the positive polarity welding electrode (FIG. 8)results in deeper weld penetration.

FIGS. 9 and 10 are cross-sectional photomicrographs of a 2.5 mm thickaluminum alloy sheet metal layer and a 3 mm thick aluminum alloy castingthat have been subjected to spot welding. FIG. 9 shows the effect ofengaging the aluminum alloy casting (top layer) with a copper alloywelding electrode that has been assigned the negative polarity andengaging the aluminum alloy sheet metal layer (bottom layer) with acopper alloy welding electrode that has been assigned the positivepolarity. Conversely, FIG. 10 shows the effect of engaging the aluminumalloy casting (top layer) with a copper alloy welding electrode that hasbeen assigned the positive polarity and engaging the aluminum alloysheet metal layer (bottom layer) with a copper alloy welding electrodethat has been assigned the negative polarity. Here, it can be seen thata better-quality spot weld was produced when the negative polaritywelding electrode, which generates less heat at its workpiece/electrodeinterface, engaged the more electrically resistive aluminum alloycasting (FIG. 9), as demonstrated by the absence of the largetriangular-shaped void formed below the interface of the casting (upperlayer) and the welding electrode that can be seen in FIG. 10.

The programmable polarity module 48 can accommodate the above-describedpolarity bias by switching the polarities of the welding electrodes 30,34, as needed, to achieve or maintain good weld quality. For instance,when spot welding a metal stack-up 12 that includes an aluminum alloysheet metal layer and a steel sheet metal layer, the polarityassignments of the two welding electrodes 30, 34 will depend on theproperties of stack-up 12 and the weld schedule, meaning that thealuminum alloy sheet metal layer could be engaged by either the positivewelding electrode or the negative welding electrode depending on thecircumstances. If the first and second welding electrodes 30, 34 aredesired to be assigned the positive and negative polarities,respectively, then the programmable polarity module 48 would be operatedin its forward polarity mode. If the opposite polarity designations aredesired, however, then programmable polarity module 48 would be operatedin its reverse polarity mode. Regarding the practice of spot welding ametal stack-up 12 that includes an aluminum alloy sheet metal layer andan aluminum alloy casting, the programmable polarity module 48 could beoperated, in many instances, in whichever mode assigns the positivepolarity to the welding electrode 30, 34 that engages the aluminum alloysheet metal layer.

The above description of preferred exemplary embodiments is merelydescriptive in nature; they are not intended to limit the scope of theclaims that follow. Each of the terms used in the appended claims shouldbe given its ordinary and customary meaning unless specifically andunambiguously stated otherwise in the specification.

The invention claimed is:
 1. A method of practicing resistance spotwelding, the method comprising: providing a direct current power supplyunit that is electrically connected to a welding gun and configured todeliver a direct welding current to the welding gun for passage betweena first welding electrode and a second welding electrode carried by thewelding gun, the direct current power supply unit comprising aprogrammable polarity module that includes a first interchangeablepolarity output lug, which electrically communicates with the firstwelding electrode, and a second interchangeable polarity output lug,which electrically communicates with the second welding electrode;locating a metal workpiece stack-up between the first and second weldingelectrodes, the metal workpiece stack-up including a first metalworkpiece and an overlapping second metal workpiece, wherein at leastone of the first metal workpiece or the second metal workpiece iscomprised of an aluminum alloy; contacting a surface of the first metalworkpiece with the first welding electrode and contacting a surface ofthe second metal workpiece with the second welding electrode;controlling the first and second interchangeable polarity output lugs ofthe programmable polarity module to assign a polarity, either positiveor negative, to the first welding electrode and to assign a polarity,either positive or negative but opposite that of the first weldingelectrode, to the second welding electrode; and forming a weld nugget ata faying interface of the first metal workpiece and the second metalworkpiece by delivering a direct welding current from the programmablepolarity module to the welding gun and passing the direct weldingcurrent between the first and second welding electrodes and through thefirst and second metal workpieces.
 2. The method set forth in claim 1,wherein the first metal workpiece is an aluminum alloy sheet metal layerand the second metal workpiece is an aluminum alloy sheet metal layer,and wherein the method comprises: assigning the polarity of the firstwelding electrode as positive and the polarity of the second weldingelectrode as negative; forming a first weld nugget at the fayinginterface of the first and second aluminum alloy sheet metal layers;switching the polarities of the first and second welding electrodes suchthat the polarity of the first welding electrode is assigned as negativeand the polarity of the second welding electrode is assigned aspositive; and forming a second weld nugget at the faying interface ofthe first and second aluminum alloy sheet metal layers.
 3. The methodset forth in claim 1, wherein the first metal workpiece is an aluminumalloy sheet metal layer and the second metal workpiece is a spotweldable material having a greater electrical resistivity than thealuminum alloy sheet metal layer, and wherein the method comprises:assigning the polarity of the first welding electrode, which contactsthe aluminum alloy sheet metal layer, as positive, and assigning thepolarity of the second welding electrode, which contacts the secondmetal workpiece, as negative.
 4. The method set forth in claim 3,wherein the second metal workpiece is a steel sheet metal layer or analuminum alloy casting.
 5. The method set forth in claim 1, whereincontrolling the first and second interchangeable polarity output lugs ofthe programmable polarity module comprises: operating a first pair ofsilicon controlled rectifiers (SCR's) associated with the firstinterchangeable polarity output lug and a second pair of SCR'sassociated with the second interchangeable polarity output lug in aforward polarity mode or a reverse polarity mode, the forward polaritymode assigning a positive polarity to the first welding electrode and anegative polarity to the second welding electrode, and the reversepolarity mode assigning a negative polarity to the first weldingelectrode and a positive polarity to the second welding electrode.
 6. Amethod of practicing resistance spot welding, the method comprising:locating a metal workpiece stack-up between a first welding electrodeand a second welding electrode, each of the first welding electrode andthe second welding electrode being carried by a welding gun having adirect current power supply unit installed thereon, the metal workpiecestack-up including a first metal workpiece and an overlapping secondmetal workpiece, wherein the direct current power supply unit comprisesa programmable polarity module that includes a first interchangeablepolarity output lug, which electrically communicates with a first gunarm of the welding gun that carries the first welding electrode, and asecond interchangeable polarity output lug, which electricallycommunicates with a second gun arm of the welding gun that carries thesecond welding electrode; contacting a surface of the first metalworkpiece with the first welding electrode and contacting a surface ofthe second metal workpiece with the second welding electrode; forming afirst weld nugget at a faying interface of the first metal workpiece andthe second metal workpiece by passing a direct welding current, which issupplied by the direct current power supply unit, between the first andsecond welding electrodes and through the first and second metalworkpieces, the first welding electrode having a polarity, eitherpositive or negative, and the second welding electrode having apolarity, either positive or negative but opposite that of the firstwelding electrode; switching the polarity of the first welding electrodeand the polarity of the second welding electrode while the directcurrent power supply unit remains installed on the welding gun; andforming a second weld nugget at the faying interface of the first metalworkpiece and the second metal workpiece by passing a direct weldingcurrent, which is supplied by the direct current power supply unit,between the first and second welding electrodes and through the firstand second metal workpieces.
 7. The method set forth in claim 6, whereinswitching the polarity of the first welding electrode and the polarityof the second welding electrode comprises: operating a first pair ofsilicon controlled rectifiers (SCR's) associated with the firstinterchangeable polarity output lug and a second pair of SCR'sassociated with the second interchangeable polarity output lug in aforward polarity mode or a reverse polarity mode, the forward polaritymode assigning a positive polarity to the first welding electrode and anegative polarity to the second welding electrode, and the reversepolarity mode assigning a negative polarity to the first weldingelectrode and a positive polarity to the second welding electrode. 8.The method set forth in claim 6, wherein the first metal workpiece is analuminum alloy sheet metal layer and the second metal workpiece is analuminum alloy sheet metal layer.
 9. A method of practicing resistancespot welding, the method comprising: providing a direct current powersupply unit that is electrically connected to a welding gun andconfigured to deliver a direct welding current to the welding gun forpassage between a first welding electrode carried on a first gun arm anda second welding electrode carried on a second gun arm, the directcurrent power supply unit having a first interchangeable polarity outputlug that electrically communicates with the first gun arm and a secondinterchangeable polarity output lug that electrically communicates withthe second gun arm; locating a metal workpiece stack-up between thefirst and second welding electrodes, the metal workpiece stack-upincluding a first metal workpiece and an overlapping second metalworkpiece; contacting a surface of the first metal workpiece with thefirst welding electrode and contacting a surface of the second metalworkpiece with the second welding electrode; assigning a polarity,either positive or negative, to the first welding electrode, andassigning a polarity, either positive or negative but opposite that ofthe first welding electrode, to the second welding electrode, bycontrolling a polarity of the first interchangeable polarity output lugassociated with the first welding electrode and a polarity of the secondinterchangeable polarity output lug associated with the second weldingelectrode; and forming a weld nugget at a faying interface of the firstmetal workpiece and the second metal workpiece by delivering a directwelding current from the direct current power supply unit to the weldinggun and passing the direct welding current between the first and secondwelding electrodes and through the first and second metal workpieces.10. The method set forth in claim 9, wherein the direct current powersupply unit comprises a transformer, a rectifier, and a programmablepolarity module, the programmable polarity module being electricallyconnected to the rectifier and the welding gun and including the firstinterchangeable polarity output lug, which electrically communicateswith the first gun arm that carries the first welding electrode, and thesecond interchangeable polarity output lug, which electricallycommunicates with the second gun arm that carries the second weldingelectrode.
 11. The method set forth in claim 10, wherein the transformerreceives an input alternating current and creates, from that inputalternating current, a secondary alternating current that has a lowervoltage and a higher amperage than the input alternating current, andthe rectifier comprises a collection of semiconductor diodes thatconverts the secondary alternating current received from the transformerinto the direct welding current for delivery to the programmablepolarity module, the welding current being delivered to the programmablepolarity module by way of a fixed positive polarity output lug and afixed negative polarity output lug, which are included on the rectifier,and a fixed positive polarity input lug and a fixed negative polarityinput lug of the module.
 12. The method set forth in claim 10, whereineach of the first interchangeable polarity output lug and the secondinterchangeable polarity output lug is composed of copper or a copperalloy.
 13. The method set forth in claim 10, wherein assigning apolarity to the first welding electrode and assigning a polarity to thesecond welding electrode comprises: controlling a first pair of siliconcontrolled rectifiers (SCR's) associated with the first interchangeablepolarity output lug and a second pair of SCR's associated with thesecond interchangeable polarity output lug to be in a forward polaritymode or a reverse polarity mode, the forward polarity mode assigning apositive polarity to the first interchangeable polarity output lug and anegative polarity to the second interchangeable polarity output lug,which correspondingly assigns a positive polarity to the first weldingelectrode and a negative polarity to the second welding electrode, andthe reverse polarity mode assigning a negative polarity to the firstinterchangeable polarity output lug and a positive polarity to thesecond interchangeable polarity output lug, which correspondinglyassigns a negative polarity to the first welding electrode and apositive polarity to the second welding electrode.
 14. The method setforth in claim 9, wherein the first metal workpiece is an aluminum alloysheet metal layer and the second metal workpiece is an aluminum alloysheet metal layer.
 15. The method set forth in claim 14, furthercomprising: switching the polarity of the first welding electrode andthe polarity of the second welding electrode while the direct currentpower supply unit remains installed on the welding gun; and forming asecond weld nugget at the faying interface of the first and second metalworkpieces.
 16. The method set forth in claim 9, wherein the first metalworkpiece is an aluminum alloy sheet metal layer and the second metalworkpiece is a spot weldable material having a greater electricalresistivity than the aluminum alloy sheet metal layer.
 17. The methodset forth in claim 16, wherein the polarity of the first weldingelectrode, which contacts the aluminum alloy sheet metal layer, isassigned as positive, and the polarity of the second welding electrode,which contacts the second metal workpiece, is assigned as negative. 18.The method set forth in claim 16, wherein the first metal workpiece isan aluminum alloy sheet metal layer and the second metal workpiece is asteel sheet metal layer or an aluminum alloy casting.